1. Field of the Invention
The present invention relates to an amplitude adjusting circuit.
2. Description of the Related Art
Through communication networks such as LANs in offices and vehicle-mounted networks, digital signals are transmitted between apparatuses in the form of signals of various formats. Communication networks are beginning to be used to connect various digital apparatuses other than computers as well as being used to connect computers and their peripherals. An example thereof is vehicle-mounted networks and, for example, a MOST (Media Oriented Systems Transport) system has been proposed as a standard for vehicle-mounted networks. In the MOST system, a ring-like vehicle-mounted network is configured, and various apparatuses such as a car navigation system, a CD/DVD player, a speaker, a display, a telephone are connected to the network. The vehicle-mounted network is used, for example, in a way that the CD/DVD player transmits a reproduced digital signal to the speaker via the vehicle-mounted network and that the speaker converts the digital signal into voice and outputs.
As a digital signal transmission method in a communication network, there are a base band method in which a digital signal is transmitted as it is and a broad band method in which an analog signal obtained by modulating a carrier wave with a digital signal is transmitted. An amplitude shift keying method as a broad band method will be described in detail below.
FIG. 8 is a diagram illustrating the configuration of a conventional ASK modulation circuit. The ASK modulation circuit of. FIG. 8 has transmit data D, that is serial digital data, inputted thereto and generates an ASK modulated signal S whose amplitude varies in response to changes in the bit value of the transmit data D over time and outputs to a network.
With reference to the waveforms of main signals of the conventional ASK modulation circuit of FIG. 9 as needed, the configuration of the ASK modulation circuit of FIG. 8 will be described.
A reference clock generator 10 generates a clock signal CL of a frequency proportional to the bit rate of the transmit data D. Let r be the bit rate (bps) of the transmit data D, then the frequency of the clock signal CL is expressed as n (natural number)×r (Hz).
Amplifiers 12, 14 each have the clock signal CL of a rectangular waveform inputted thereto and have their output amplitude level decided by a predetermined gain. For example, the amplifiers 12, 14 produce respective clock signals CL1, CL2 of a rectangular waveform that swing to their peak and bottom with ground potential (zero level) as their reference (see (a), (b) of FIG. 9). Note that the clock signals CL1, CL2 are set to have amplitude levels different from each other.
A switching controller 20 latches the bit value of the transmit data D synchronously with the clock signal CL and depending on the bit value, generates a control signal SW to control the on/off of switches 16, 18 (see (c) of FIG. 9).
The switches 16, 18 switch on/off complimentarily according to the control signal SW supplied from the switching controller 20. For example, when the control signal SW is at a High (H) level, the switch 16 is off and the switch 18 is on. When the control signal SW is at a Low (L) level, the switch 16 is on and the switch 18 is off. The outputs of the switches 16, 18 are combined and input to an LPF 22.
The LPF 22 removes the high frequency component from the combined signal of the outputs of the switches 16, 18 (see (d) of FIG. 9) and produces the ASK modulated signal S of a smooth sine waveform (see (e) of FIG. 9).
Such a conventional ASK modulation circuit is disclosed in, for example, Japanese Patent Laid-Open Publication No. 2001-119442).
With circuits like the amplifiers 12, 14 of FIG. 8 that set their output amplitude level (hereinafter called conventional “amplitude setting circuits”), there is the problem that their gain varies with temperature because circuit elements thereof have a temperature characteristic. As a result, although its amplitude level is set, the output signal varies in amplitude with temperature. Furthermore, to change the amplitude level of the output signal, the amplifier needs to be replaced by an amplifier having a gain corresponding to the new amplitude level.
Hence, instead of a fixed-gain amplifier like the amplitude setting circuit, a circuit whose output amplitude level is adjustable arbitrarily through the choice of resistances such as a resistor-ladder electronic volume or an attenuator (hereinafter called a conventional “amplitude adjusting circuit”) is often adapted. However, it is known that the conventional amplitude adjusting circuit is easy to be affected by temperature variation like the conventional amplitude setting circuit is, thus having large variation in its output amplitude level. Moreover, since the conventional amplitude adjusting circuit has its output amplitude level adjusted through the choice of a finite number of resistances, there is a great restriction on fine adjustment of the amplitude level, and also there is the problem that it is difficult to arbitrarily select the operation reference voltage of a circuit at the rear stage. Further, the conventional amplitude adjusting circuit is complex in configuration, thus causing the problem that the scale of the entire system becomes large.